Method for making a photocatalyst nanocomposite

ABSTRACT

An efficient photocatalyst nanocomposite comprising reduced graphene oxide, noble metal, and a metal oxide prepared by a one-step method that utilizes date seed extract as a reducing and nanoparticle determining size agent. The photocatalyst of the invention is a more effective sunlight photocatalyst than that prepared by traditional method in the photo decomposition of organic compounds in contaminated water.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of U.S. application Ser. No.16/437,614, now allowed, having a filing date of Jun. 11, 2019.

BACKGROUND OF THE INVENTION Field of the Invention

The invention pertains to methods for green synthesis of photocatalystnanocomposites comprising a noble metal, a metal oxide, and reducedgraphene oxide, compositions containing the photocatalystnanocomposites, heterojunction structures based on the photocatalystnanocomposites, and methods of purifying water using the photocatalystnanocomposites to remove soluble organic contaminants in combinationwith sunlight.

Description of Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. All references citedherein are incorporated by reference. Work of the presently namedinventors, to the extent it is described in this background section, aswell as aspects of the description which may not otherwise qualify asprior art at the time of filing, are neither expressly or impliedlyadmitted as prior art against the present disclosure.

Zinc oxide nanostructures have several properties making them promisingmaterials for use in several advanced technologies and applications [Miret al. “Preparation of ZnO NanoFlowers and Zn Glycerolate NanoplatesUsing Inorganic Precursors via a Convenient Rout and Application in DyeSensitized Solar Cells” Chem. Eng. J. 181 (2012) 779-89;Salavati-Niasari et al. “Preparation of ZnO Nanoparticles from[bis(acetylacetonato)zinc(II)]-oleylamine Complex by ThermalDecomposition” Mater. Lett. 62 (2008) 1890-1892; Mohamed, H. H.“Sonochemical synthesis of ZnO hollow microstructure/reduced grapheneoxide for enhanced sunlight photocatalytic degradation of organicpollutants” J. Photochem. Photobiol. A 353 (2018) 401-408;Salavati-Niasari et al. “Nanosphericals and Nanobundles of ZnO:Synthesis and Characterization” J. Alloys Compd. 509 (2011) 61-65; andSalavati-Niasari et al. “Synthesis and Characterization of ZnONanocrystals from Thermolysis of New Precursor” Chem. Eng. J. 146 (2009)498-502]. Both physical and chemical methods have been developed for thepreparation of ZnO nanoparticles [Jamal et al. “Optical Properties ofNanostructured ZnO Prepared by a Pulsed Laser Deposition Technique”Mater. Lett. 132 (2014) 31-33; Djurišić et al. “Optical Properties ofZnO Nanostructures” Small 2 (2006) 944-961; Ogata et al. “Control ofChemical Bonding of the ZnO Surface Grown by Molecular Beam Epitaxy”Appl. Surf. Sci. 237 (2004) 348-351; and Zhao et al. “Ferromagnetic ZnONanoparticles Prepared by Pulsed Laser Deposition in Liquid” Mater Lett.85 (2012) 164-167]. Precipitation, hydrolysis, pyrolysis, hydrothermal,sol-gel, and sonochemical are some of the chemical methodologies usedfor the synthesis of ZnO nanoparticles having various sizes andmorphologies [Debanath et al. “Study of Blueshift of Optical BandGap inZinc Oxide (ZnO) Nanoparticles Prepared by Low-Temperature Wet ChemicalMethod” Mater. Lett. 111 (2013) 116-119; Li et al. “One-Step,Solid-State Reaction to ZnO Nanoparticles in the Presence of IonicLiquid” Mater. Sci. Semicond. Process. 14 (2011) 184-187; Raja et al.“Synthesis, Structural and Optical Properties of ZnO and Ni-Doped ZnOHexagonal Nanorods by Co-Precipitation Method” Spectrochim. Acta Part AMol. Biomol. Spectrosc. 120 (2014) 19-24; Kuan et al. “ZnO MorphologyDevelopment Controlled by Tuning the Hydrolysis Process” J. Cryst.Growth 372 (2013) 213-218; Lee et al. “Synthesis and PhotocatalyticProperty of ZnO Nanoparticles Prepared by Spray-Pyrolysis Method” Phys.Procedia. 32 (2012) 320-326; Maryanti et al. “Synthesis of ZnOnanoparticles by hydrothermal method in aqueous rinds extracts ofSapindus rarak DC” Mater. Lett. 118 (2014) 96-98; Li et al. “MicrowaveHydrothermal Synthesis of K Doped ZnO Nanoparticles with EnhancedPhotocatalytic Properties Under Visible-Light”. Mater. Lett. 118 (2014)17-20;Omri et al. “Effects of Temperature on the Optical and ElectricalProperties of ZnO Nanoparticles Synthesized by Sol-Gel Method”Microelectron. Eng. 128 (2014) 53-58; and Ba-Abbad et al. “The Effect ofProcess Parameters on the Size of ZnO Nanoparticles Synthesized Via theSol-Gel Technique” J. Alloy. Compd. 550 (2013) 63-70]. The size andmorphology of ZnO nanoparticles can be controlled by optimizing thetemperature, pH, and reaction medium [Mallika et al. “Synthesis andOptical Characterization of Aluminium Doped ZnO Nanoparticles” Ceram.Int. 40 (2014) 12171-12177; and Niasari et al. “Nanosphericals andNanobundles of ZnO: Synthesis and Characterization” J. Alloy. Compd. 509(2011) 61-65].

Currently, green and environmentally friendly methods for the synthesisof nano-sized materials are highly desirable. Methods utilizing plantextracts as reducing/oxidizing agents, stabilizers, and capping agentshave been proven successful in producing crystallized nanoparticles[Ghodake et al. “ Pear Fruit Extract-Assisted Room-TemperatureBiosynthesis of Gold Nanoplates” Colloids Surf B. 75 (2010) 584-589; andEdison et al. “Instant Green Synthesis of Silver Nanoparticles UsingTerminaliachebula Fruit Extract and Evaluation of Their CatalyticActivity on Reduction of Methylene Blue” Process Biochem. 47 (2012)1351-1357].

Single ZnO nanostructures have been shown to be marginally efficientsolar photocatalyst because of high electron-hole recombination and alimited visible light-harvesting capability, which making themphotoactive catalysts only in the UV region [Yuan et al.“Hetero-Nanostructured Suspended Photocatalysts for Solar-to-FuelConversion” Energy Environ. Sci. 7 (2014) 3934-3951; and Chen et al.“Semiconductor-Based Photocatalytic Hydrogen Generation” Chem. Rev. 110(2010) 6503-6570]. To overcome these limitations, hybrid nanostructuresmade of two or more functional components have been proposed.Photocatalysts having a hetero-structure structure significantlydecrease the recombination of photogenerated charge carries. Inaddition, such photocatalysts expand the light absorption to the visibleregion. Therefore, substantial enhancement of solar photocatalyticefficiency is achieved through the hetero-structured photocatalyst[Mayer et al. “Forming Heterojunctions at the Nanoscale for ImprovedPhotoelectrochemical Water Splitting by Semiconductor Materials: CaseStudies on Hematite” Acc. Chem. Res. 46 (2013) 1558-1566; Jang et al.“Heterojunction Semiconductors: A Strategy to Develop EfficientPhotocatalytic Materials for Visible Light Water Splitting” Catal.Today. 185 (2012) 270-277; Wang et al. “Visible Light Driven Type IIHeterostructures and Their Enhanced Photocatalysis Properties: A Review,Nanoscale” 5 (2013) 8326-8339; and Fan et al. “Semiconductor-BasedNanocomposites for Photocatalytic H₂ Production and CO₂ Conversion,PhysChemChemPhys. 15 (2013) 2632-2649].

Incorporation of noble metal nanoparticles/ZnO heterojunction is anefficient approach to reduce the charge carrier recombination rate andhence, enhance the photocatalytic activity. The key concept inmetal-semiconductor structure is Schottky junction that is created dueto the close contact of a metal and n-type semiconductor. Schottkyjunctions facilitate the transfer of photo-generated electrons to thecontacting metal. In such process, the metal acts as an electron trapcenter and accelerates the separation of photo-generated charges.Thereby, a hole will have more time to react as an oxidizer on thesemiconductor surface. Moreover, the metal surface provides active sitesfor a reduction reaction [Yang et al. “Roles of Cocatalysts inPhotocatalysis and Photoelectrocatalysis” Acc. Chem. Res. 46 (2013)1900-1909]. In this manner various noble metals have been used toconstruct metal-semiconductor hetero-structure such as Pt, Pd and Au[Lingampalli et al. “Highly Efficient Photocatalytic Hydrogen Generationby Solution-Processed ZnO/Pt/CdS, ZnO/Pt/Cd1-xZnxS and ZnO/Pt/CdS1-xSexHybrid Nanostructures” Energy Environ. Sci. 6 (2013) 3589 -3594; Wu etal. “Pd-Gardenia-TiO₂ a Photocatalyst for H₂ Evolution from Pure Water”Int. J. Hydrogen Energy 37 (2012) 109-114; Jin et al. “PhotocatalyticActivities Enhanced for Decompositions of Organic Compounds overMetal-Photodepositing Titanium Dioxide” Chem. Eng. J. 97 (2004) 203-211;and Chiarello et al. “Photocatalytic Hydrogen Production over FlameSpray Pyrolysis-Synthesized TiO2 and Au/TiO2” Appl. Catal. B Environ. 84(2008) 332-339].

CN106582717A discloses a method of preparing a grapheneoxide-CdS-ZnO-porous silicon composite as a photocatalyst fordegradation of dyes such as methyl orange. The disclosed catalyst ischemically distinct from that of the present disclosure and does notcontain a noble metal, a metal oxide, and reduced graphene oxidenanoparticles.

CN103920442A discloses a photoelectric catalyst comprising a catalyticfilm deposited on an electrically conducting substrate. Thephotoelectric catalyst is a component of a device in which the catalystis connected to a thermoelectric device having a photoconductingmaterial. The disclosed catalytic material includes ZnO but does notdisclose a photocatalyst nanocomposite comprising a noble metal, a metaloxide, and reduced graphene oxide nanoparticles.

KR1453391B1 discloses a photocatalytic method for degrading dye using ametal/metal oxide catalyst supported on carbon. The disclosed metal isgold or silver and the metal oxide is titanium dioxide, cerium dioxide,zinc oxide, or tin oxide. The disclosed carbon support is a carbonpaper, a carbon cloth, a carbon felt, an activated carbon, a graphiteroad, or non-woven graphite. The use of a catalyst comprising a reducedgraphene oxide is not described.

CN108479756A discloses a method of preparation and use of a Bi₂WO₆photocatalyst. The CN108479756A catalyst is not a composite comprising anoble metal, a metal oxide, and reduced graphene.

CN106311196B discloses the preparation of tubular titanium dioxidenanoparticles and use thereof as a photocatalyst. The CN106311196Bcatalyst does not contain a noble metal, a metal oxide, and reducedgraphene oxide.

CN103991921A discloses a floating adsorption bed comprising aphotocatalytic layer, a nitrogen absorbing layer, and a magneticdephosphorized layer supported on a substrate. The photocatalytic layeris graphene. The patent reference does not disclose and the use of agreen reduced graphene oxide nanoparticles.

Soliman et al. [J. Water Proc. Engin. (2017) 245-255] disclose thepreparation of activated carbon from date palm stone which is loadedwith Zn(II), Cd(II), Cu(II), and Pd(II) metal ions by adsorption and ionexchange. The activated carbon displays photocatalytic activity for thedegradation of crystal violet dye. The most photo-catalytically activematerial observed was a Zn(II)-loaded activated carbon but does notcontain a noble metal, a metal oxide, and reduced graphene oxidenanoparticles.

Zayed et al. [Spectrochimica ACTA Part A: Mol. Biomol. Spect. (2014)121, 238-244] disclose the preparation of gold nanoparticles usingPhoenix dactylifera leaf extract. The reference does not mentionnanocomposite comprising zinc oxide and reduced graphene oxide from dateseed extract.

Lee et al. [Mat. Chem. Phys. (2015) https://www. sciencedirect.com/science/article/pii/S0254058415302765] disclose the preparation of aphotocatalyst composite comprising gold, graphene oxide, and zinc oxide.Au/GO/ZnO. The method of preparation comprises the preparation of ZnOand gold nanoparticles separately as well as graphene oxide, andpreparing a Au/GO composite followed by decorating with ZnO. Theresulting photocatalyst does not contain reduced graphene oxide.

Blaisi et al. [https://app.dimensions.ai/details/publication/pub.1107476191] disclose the preparation of a composite comprising date palmash and a MgAl-layered double hydroxide and the use thereof in theremoval of methyl orange from an aqueous medium.

It is one objective of the present disclosure to provide an efficientphotocatalyst nanocomposite comprising a noble metal, a metal oxide, andreduced graphene oxide prepared by a green hydrothermal method utilizinga naturally derived carbon source such as date seed extract (PDE) usedas a reducing agent in a single step reaction. The photocatalyst of theinvention is efficient in catalyzing the photo-decomposition of dyes incontaminated water.

SUMMARY OF THE INVENTION

A first aspect of the invention is directed to a photocatalystnanocomposite comprising nanoparticles of a noble metal, a metal oxide,and reduced graphene oxide (rGO), wherein:

-   -   the photocatalyst nanocomposite comprises rGO in an amount in        the range of 40 wt. % to 60 wt. % of the total weight of the        photocatalyst nanocomposite,    -   the noble metal nanoparticles having a dimeter in the range of 5        nm to 50 nm are dispersed on the metal oxide nanoparticles        having dimeter in the range of 20 nm to 80 nm in an amount in        the range of 2 mol. % to 10 mol. % of the molar amount of the        metal oxide; and    -   the metal oxide is an oxide of a metal selected from the group        consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, and        Mo.

In a preferred embodiment, the composite comprises rGO in an amount inthe range of 48 wt. % to 52 wt. % of the total weight of thenanocomposite.

In another preferred embodiment, the noble metal nanoparticles have adiameter in the range of 10-30 nm.

In another preferred embodiment, the noble metal is selected from thegroup consisting of ruthenium, rhodium, palladium, silver, osmium,iridium, platinum, and gold.

In a more preferred embodiment, the noble metal is gold.

In another preferred embodiment, the metal oxide is selected from thegroup consisting of Ti, V, M, and Zn.

In a more preferred embodiment, the metal oxide is zinc oxide.

In another more preferred embodiment, the zinc oxide is hexagonalwurtzite zinc oxide.

In another preferred embodiment, the metal oxide nanoparticles havediameter in the range of 25-50 nm.

In another preferred embodiment, the photocatalyst nanocompositecomprises about 5 mol. % of gold of the molar amount of the zinc oxide.

In some preferred embodiments, photocatalyst nanocomposite comprisinggold, zinc oxide, and reduced graphene oxide nanoparticles (rGO),wherein the photocatalyst nanocomposite comprises rGO in an amount inthe range of 45 wt. % to 55 wt. % of the total weight of thephotocatalyst nanocomposite and the gold nanoparticles having dimeter inthe range of 10 nm to 30 nm are dispersed on hexagonal wurtzite zincoxide having dimeter in the range of 20 nm to 60 nm in an amount in therange of 2 mol. % to 10 mol. % of the amount of the zinc oxide.

In another preferred embodiment, the composite comprising gold, zincoxide, and reduced graphene oxide nanoparticles in an amount of about 49wt. %, 45.5 wt. %, and 5.5 wt. %, respectively, of the total weight ofthe composite.

Other preferred embodiments of the invention are directed to aphotocatalyst nanocomposite comprising nanoparticles of a noble metal, ametal oxide, and reduced graphene oxide (rGO), wherein:

-   -   the photocatalyst nanocomposite comprises rGO in an amount in        the range of 0.1 wt. % to 10 wt. % of the total weight of the        photocatalyst nanocomposite,    -   the noble metal nanoparticles having a dimeter in the range of 5        nm to 50 nm are dispersed on the metal oxide nanoparticles        having dimeter in the range of 20 nm to 80 nm in an amount in        the range of 2 mol. % to 10 mol. % of the molar amount of the        metal oxide; and    -   the metal oxide is an oxide of a metal selected from the group        consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, and        Mo.

In a preferred embodiment, the composite comprises rGO in an amount inthe range of 1.8 wt. % to 2.5 wt. % of the total weight of thenanocomposite.

In some preferred embodiments, photocatalyst nanocomposite comprisinggold, zinc oxide, and reduced graphene oxide nanoparticles (rGO),wherein the photocatalyst nanocomposite comprises rGO in an amount inthe range of 1.8 wt. % to 2.5 wt. % of the total weight of thephotocatalyst nanocomposite and the gold nanoparticles having dimeter inthe range of 10 nm to 30 nm are dispersed on hexagonal wurtzite zincoxide having dimeter in the range of 20 nm to 60 nm in an amount in therange of 2 mol. % to 10 mol. % of the amount of the zinc oxide.

In another preferred embodiment, the composite comprising gold, zincoxide, and reduced graphene oxide nanoparticles in an amount of about 49wt. %, 49.1 wt. %, and 1.8 wt. %, respectively, of the total weight ofthe composite.

A second aspect of the invention is directed to a method of making thephotocatalyst nanocomposite of the invention comprising:

-   -   preparing an aqueous suspension of metal salt, a noble metal        precursor, and graphene,    -   mixing the aqueous suspension with date seed extract to form a        mixture, and    -   heating the mixture at temperature in the range of 100-200° C.        for a time in the range of 10 to 20 hours;    -   wherein the date seed extract is prepared by grinding date seed        and heating the ground date seed in water at a temperature in        the range of 60 to 100° C. for a time in the range of 1 to 5        hours and separating the solid from the liquid extract.

In a preferred embodiment, the molar amount of the noble metal precursoris in the range of 1 mol. % to 10 mol. % of the molar amount of themetal salt.

In another preferred embodiment, the metal salt is zinc acetate and thenoble metal precursor is chloroauric acid (HAuCl₄).

In a preferred embodiment, the amount of graphene in the suspension isthe range of 2 wt. % to 60 wt. % of the total weight of thephotocatalyst nanocomposite.

In a preferred embodiment, the amount of graphene in the suspension isthe range of 0.1 wt. % to 10 wt. % of the total weight of thephotocatalyst nanocomposite.

In a preferred embodiment, the molar amount of the noble metal precursoris about 5 mol. % of the molar amount of the metal salt.

A third aspect of the invention is directed to a method of preparing acomposition comprising noble metal nanoparticles comprises:

-   -   preparing an aqueous solution or suspension comprising a noble        metal precursor,    -   mixing the aqueous suspension with date seed extract to form a        mixture, and    -   heating the mixture at temperature in the range of 100-200° C.        for a time in the range of 10 to 20 hours;    -   wherein the date seed extract is prepared by grinding date seed        and heating the ground date seed in water at a temperature in        the range of 60 to 100° C. for a time in the range of 1 to 5        hours and separating the solid from the liquid extract, and        separating the solid from the liquid extract, and the noble        metal nanoparticles have a dimeter in the range of 5 nm to 30        nm.

A fourth aspect of the invention is directed to a method ofphotodecomposition of an organic compound comprises:

-   -   contacting the photocatalyst nanocomposite of the invention with        an aqueous solution of an organic compound to form a mixture,        and    -   irradiating the mixture with sunlight.

In a preferred embodiment, the organic compound is a dye.

In a more preferred embodiment, the dye is methyl orange.

A fifth aspect of the invention is directed to a process of purifyingcontaminated water with organic materials comprises:

-   -   contacting the contaminated water with the photocatalyst        nanocomposite of invention, and    -   irradiating the mixture with sunlight.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A shows XRD pattern of the date seed extract crystals.

FIG. 1B shows FTIR spectrum of the date seed extract crystals.

FIG. 2 shows examples of the chemical structures of flavonoids andpolyphenolic compounds in date seed extract.

FIG. 3A compares the XRD patterns of GO, ZnO (Z), ZnO/rGO (ZG) andAu/ZnO/rGO (AZG) nanocomposites prepared in the presence of date seedextract (DSE).

FIG. 3B compare are the XRD patterns of ZnO/rGO (ZG) and Au/ZnO/rGO(AZG) nanocomposites in the absence of DSE.

FIG. 4 shows Raman Spectra of ZnO/rGO (ZG) and Au/ZnO/rGO (AZG)nanocomposites prepared in the presence of DSE.

FIG. 5A shows a SEM image of ZnO/rGO nanocomposites.

FIG. 5B shows a SEM image of Au/ZnO/rGO nanocomposites.

FIG. 6A shows TEM images of ZnO at 1× magnification prepared in thepresence of DSE.

FIG. 6B shows TEM images of ZnO at 4× magnification prepared in thepresence of DSE.

FIG. 6C shows TEM images of ZnO at 10× magnification prepared in thepresence of DSE.

FIG. 6D shows TEM images of ZnO/rGO at 1× magnification prepared in thepresence of DSE.

FIG. 6E shows TEM images of ZnO/rGO at 4× magnification prepared in thepresence of DSE.

FIG. 6F shows TEM images of pure Au/ZnO/rGO nanocomposites at 1×magnification prepared in the presence of DSE.

FIG. 6G shows TEM images of Au/ZnO/rGO nanocomposites at 4×magnification prepared in the presence of DSE.

FIG. 6H shows TEM images of ZnO at 2× magnification prepared in absenceof DSE.

FIG. 6I shows TEM images of ZnO/rGO at 2× magnification prepared inabsence of DSE.

FIG. 6J shows TEM images of Au/ZnO/rGO at 4× magnification prepared inabsence of DSE.

FIG. 7A shows EDX analysis of ZnO (Z DS) prepared in the presence ofDSE.

FIG. 7B shows EDX analysis of ZnO/rGO (ZG DS) prepared in presence ofDSE.

FIG. 7C shows EDX analysis of Au/ZnO/rGO nanocomposites (AZG DS)prepared in the presence of DSE.

FIG. 8A shows a time dependent UV-vis absorption spectra of an aqueoussolution of MO dye during irradiation in the presence of Au/ZnO/rGO.

FIG. 8B shows time dependent photocatalytic degradation of MO as afunction of time at pH 5.0 using different nanomaterial photocatalysts.

FIG. 9 shows a schematic representation of single step green synthesisof Au/ZnO/rGO nanocomposite.

FIG. 10 shows schematic representation of single step green synthesis ofAu/ZnO/rGO Nanocomposite.

DETAILED DESCRIPTION

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown. The presentdisclosure will be better understood with reference to the followingdefinitions.

All publications mentioned herein are incorporated herein by referencein full for the purpose of describing and disclosing the methodologies,which are described in the publications, which might be used inconnection with the description herein. Nothing herein is to beconstrued as an admission that the inventors are not entitled toantedate such disclosure by virtue of prior disclosure. Also, the use of“or” means “and/or” unless stated otherwise. Similarly, “comprise,”“comprises,” “comprising” “include,” “includes,” and “including” areinterchangeable and not intended to be limiting.

Unless otherwise specified, “a” or “an” means “one or more”.

Terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention.

The headings (such as “Background” and “Summary”) and sub-headings usedherein are intended only for general organization of topics within thepresent invention, and are not intended to limit the disclosure of thepresent invention or any aspect thereof. In particular, subject matterdisclosed in the “Background” may include novel technology and may notconstitute a recitation of prior art. Subject matter disclosed in the“Summary” is not an exhaustive or complete disclosure of the entirescope of the technology or any embodiments thereof. Classification ordiscussion of a material within a section of this specification ashaving a particular utility is made for convenience, and no inferenceshould be drawn that the material must necessarily or solely function inaccordance with its classification herein when it is used in any givencomposition.

It will be further understood that the terms “comprises” and/or“comprising,” when used in this specification, specify the presence ofstated features, steps, operations, elements, and/or components, but donot preclude the presence or addition of one or more other features,steps, operations, elements, components, and/or groups thereof.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items and may be abbreviated as“/”.

Links are disabled by deletion of http: or by insertion of a space orunderlined space before www. In some instances, the text available viathe link on the “last accessed” date may be incorporated by reference.

As used herein in the specification and claims, including as used in theexamples and unless otherwise expressly specified, all numbers may beread as if prefaced by the word “substantially”, “about” or“approximately,” even if the term does not expressly appear. As usedherein, the term “about” refers to an approximate number within 20% of astated value, preferably within 15% of a stated value, more preferablywithin 10% of a stated value, and most preferably within 5% of a statedvalue. For example, if a stated value is about 8.0, the value may varyin the range of 8±1.6, ±1.0, ±0.8, ±0.5, ±0.4, ±0.3, ±0.2, or ±0.1.

Disclosure of values and ranges of values for specific parameters (suchas temperatures, molecular weights, weight percentages, etc.) are notexclusive of other values and ranges of values useful herein. It isenvisioned that two or more specific exemplified values for a givenparameter may define endpoints for a range of values that may be claimedfor the parameter. For example, if Parameter X is exemplified herein tohave value A and also exemplified to have value Z, it is envisioned thatparameter X may have a range of values from about A to about Z.Similarly, it is envisioned that disclosure of two or more ranges ofvalues for a parameter (whether such ranges are nested, overlapping ordistinct) subsume all possible combination of ranges for the value thatmight be claimed using endpoints of the disclosed ranges. For example,if parameter X is exemplified herein to have values in the range of 1-10it also describes subranges for Parameter X including 1-9, 1-8, 1-7,2-9, 2-8, 2-7, 3-9, 3-8, 3-7, 2-8, 3-7, 4-6, or 7-10, 8-10 or 9-10 asmere examples. A range encompasses its endpoints as well as valuesinside of an endpoint, for example, the range 0-5 includes 0, >0, 1, 2,3, 4, <5 and 5.

As used herein, the words “preferred” and “preferably” refer toembodiments of the technology that afford certain benefits, undercertain circumstances. However, other embodiments may also be preferred,under the same or other circumstances. Furthermore, the recitation ofone or more preferred embodiments does not imply that other embodimentsare not useful, and is not intended to exclude other embodiments fromthe scope of the technology.

As referred to herein, all compositional percentages are by weight ofthe total composition, unless otherwise specified. As used herein, theword “include,” and its variants, is intended to be non-limiting, suchthat recitation of items in a list is not to the exclusion of other likeitems that may also be useful in the materials, compositions, devices,and methods of this technology. Similarly, the terms “can” and “may” andtheir variants are intended to be non-limiting, such that recitationthat an embodiment can or may comprise certain elements or features doesnot exclude other embodiments of the present invention that do notcontain those elements or features.

The description and specific examples, while indicating embodiments ofthe technology, are intended for purposes of illustration only and arenot intended to limit the scope of the technology. Moreover, recitationof multiple embodiments having stated features is not intended toexclude other embodiments having additional features, or otherembodiments incorporating different combinations of the stated features.Specific examples are provided for illustrative purposes of how to makeand use the compositions and methods of this technology and, unlessexplicitly stated otherwise, are not intended to be a representationthat given embodiments of this technology have, or have not, been madeor tested.

A first aspect of the invention is directed to a photocatalystnanocomposite comprising nanoparticles of a noble metal, nanoparticlesof a metal oxide, and a reduced graphene oxide (rGO) support. Thephotocatalyst nanocomposite of the invention is prepared by a methodutilizing a date seed extract as a reducing and nanoparticle sizedetermining agent. The examples herein show that the photocatalystnanocomposite is unexpectedly more efficient than that prepared by aconventional method.

As used herein, the term “noble metal” is a metal selected from thegroup consisting of ruthenium (Ru), rhodium (Rh), palladium (Pd), silver(Ag), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au). Thephotocatalyst nanocomposite may contain nanoparticles of one or more ofthe noble metals such that the nanoparticles have a diameter in therange of 1-60 nm, preferably, 2-50 nm, preferably 5-40 nm, preferably10-30 nm, preferably 15-25 nm, preferably 15-20 nm. In some embodimentof the invention, the noble metal nanoparticles are gold having adimeter in the range of 10 nm to 30 nm, preferably, 15 nm to 25, andpreferably 15 nm to 20 nm. The amount of noble metal in thenanocomposite is in the range of 1 wt. % to 15 wt. %, preferably in therange of 2 wt. % to 12 wt. %, preferably in the range of 3 wt. % to 10wt. %, preferably in the range of 4 wt. % to 8 wt. %, and preferably inthe range of 5 wt. % to 7 wt. % of the total weight of thenanocomposite.

The noble metal nanoparticles are dispersed on the metal oxidenanoparticles and the reduced graphene oxide support. As used herein theword “diameter” refers to the average diameter of a nanoparticlemeasured by transmission electron microscopy (TEM.) The metal oxidenanoparticles have a diameter in the range of 20 nm to 80 nm, preferablyin the range of 25 nm to 60 nm, preferably in the range of 30 nm to 40nm. The metal oxide may be an oxide of an element such as, but notlimited to, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, and Mo. Insome preferred embodiments of the invention, the metal oxide is selectedfrom the group consisting of Ti, V, Cr, Mn, Fe, Co, and Ni. Preferablythe metal oxide is that of Zn. Zinc oxide crystallizes in two mainforms, hexagonal wurtzite and cubic zinc blend. The wurtzite is the moststable and preferred form in the photocatalyst nanocomposite of theinvention.

In preferred embodiments, the amount of the noble metal is in the rangeof 1 mole % to 15 mole %, preferably in the range of 2 mole % to 12 mole%, preferably in the range of 4 mole % to 10 mole %, preferably in therange of 5 mole % to 8 mole %, and preferably about 5.5 mol. % based onthe molar amount of the metal oxide in the nanocomposite.

The photocatalyst nanocomposite of the invention comprises reducedgraphene oxide in an amount in the range of 30 wt. % to 70 wt. %,preferably in the range of 35 wt. % to 65 wt. %, preferably in the rangeof 40 wt. % to 60 wt. %, preferably in the range of 45 wt. % to 55 wt.%, preferably in the range of 48 wt. % to 52 wt. % of the total weightof the photocatalyst nanocomposite. In a particularly preferredembodiment, the reduced graphene oxide is about 49 wt. % of the totalweight of the photocatalyst nanocomposite.

In a particularly preferred embodiment, the photocatalyst nanocompositecomprises gold, zinc oxide, and reduced graphene oxide nanoparticles(rGO), wherein the photocatalyst nanocomposite comprises rGO in anamount in the range of 45 wt. % to 55 wt. %, preferably in the range of48 wt. % 52 wt. %, preferably about 49 wt % of the total weight of thephotocatalyst nanocomposite and gold nanoparticles having a diameter inthe range of 5 nm to 35 nm, preferably in the range 10 nm to 30 nm, andpreferably in the range of 15 nm to 20 nm which are dispersed onhexagonal wurtzite zinc oxide nanoparticles having a diameter in therange of 20 nm to 60 nm, preferably in the range of 25 nm to 50 nm,preferably in the range of 30 nm to 40 nm and the gold nanoparticles arepresent in an amount in the range of 2 mol. % to 10 mol. % of the molaramount of the zinc oxide.

In some other embodiments, the photocatalyst nanocomposite of theinvention comprises reduced graphene oxide in an amount in the range of0.1 wt. % to 10 wt. %, preferably in the range of 0.5 wt. % to 8 wt. %,preferably in the range of 1.0 wt. % to 6 wt. %, preferably in the rangeof 1.8 wt. % to 4 wt. %, preferably in the range of 2.0 wt. % to 3 wt. %of the total weight of the photocatalyst nanocomposite. In aparticularly preferred embodiment, the reduced graphene oxide is about 2wt. % of the total weight of the photocatalyst nanocomposite.

In a particularly preferred embodiment, the photocatalyst nanocompositecomprises gold, zinc oxide, and reduced graphene oxide nanoparticles(rGO), wherein the photocatalyst nanocomposite comprises rGO in anamount in the range of 1.5 wt. % to 3.0 wt. %, preferably in the rangeof 1.8 wt. % 2.5 wt. %, preferably about 1.9 wt% of the total weight ofthe photocatalyst nanocomposite and gold nanoparticles having a diameterin the range of 5 nm to 35 nm, preferably in the range 10 nm to 30 nm,and preferably in the range of 15 nm to 20 nm which are dispersed onhexagonal wurtzite zinc oxide nanoparticles having a diameter in therange of 20 nm to 60 nm, preferably in the range of 25 nm to 50 nm,preferably in the range of 30 nm to 40 nm and the gold nanoparticles arepresent in an amount in the range of 2 mol. % to 10 mol. % of the molaramount of the zinc oxide.

The second aspect of the invention is directed to a method of preparinga noble metal nanoparticle or composite thereof such as, but not limitedto the photocatalyst nanocomposite of the invention. The methodcomprises:

-   -   preparing a mixture of an aqueous solution or suspension        comprising a noble metal precursor and date seed extract, and    -   heating the mixture at temperature in the range of 100-200° C.        for a time in the range of 5 to 30 hours.

Any date seed from any location may be utilized in obtaining the dateseed extract. The date seed extract is prepared from seeds by washingwith water, drying, and grinding the date seed to a fine powder. Theseed powder is suspended in water in an amount in the range 50 to 500g/L, preferably 100 to 400 g/L, preferably 250-350 g/L, and preferablyabout 300 g/L. The date seed powder suspension is heated at atemperature in the range of 60-100° C., preferably in the range of70-90° C., preferably about 80° C. for a time in the range 5-50 h,preferably 10-30 hours, preferably about 15 h. The suspension isfiltered to remove the solid material and the filtrate is refrigeratedat about ° C. until used.

The noble metal precursor may be any salt or compound of a noble metalsuch as but not limited to AuF₃, AuF₅, (AuCl₃)₂, Au₄Cl₈, AuBr, AuBr₃,AuI, AuI₃, Au₂O₃, Au₂S, Au₂S₃, HAuCl₄, PtCl₂, PtCl₄, PtF₄, PtO₂,H₂PtCl₆, palladium halide including but not limited to PdF₂, PdF₃, PdF₄,PdF₆, Pd(PdF₆), palladium acetate, palladium acetylacetonate, PdCl₂,PdBr₂, palladium cyanide, Pd(NO₃)₂, Ag₂CO₃, AgF₂, AgNO₃, silver acetate,O_(X)O₄, O_(S)O₂, RuO₂, RUO₄, K₂RUO₄, RuCl₃, RuF₃, RhCl₃, RhF₆, RhO₂,Na₂RhO₃, IrO₂, Ir₂O₃,IrO₄, IrCl₂, IrCl₃, and the like.

In some embodiments of the method, the mixture further comprisesgraphene oxide and a metal salt as defined herein above to produce thephotocatalyst of the invention. Any suitable metal salt such as that of,but not limited to, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, and Momay be utilized. Examples of the salts include, but are not limited tohalides, carboxylate, carbonate, alkoxide and the like such as, but notlimited to ZnCl₂, ZnBr₂, zinc acetate, zinc isoprpoxide, Zn(OCH₃)₂,titanium isopropoxide, ZrCl₄, Zr₄(OC₂H₅)₁₆, FeCl₂, FeCl₃, TiCl₄,Ti₄(OC₂H₅)₁₆, VCl₃, VBr₃, vanadium acetate, vanadium oxytriisopropoxide,MnCl₂, MnBr₂, manganese acetate, and the like.

The amount of the noble metal in the suspension is in the range of 1mole % to 15 mole %, preferably in the range of 2 mole % to 12 mole %,preferably in the range of 4 mole % to 10 mole %, and preferably in therange of 5 mole % to 8 mole % of the metal oxide in the nanocomposite.The amount of graphene oxide in the suspension is in the range of 30 wt.% to 70 wt. %, preferably in the range of 35 wt. % to 65 wt. %,preferably in the range of 40 wt. % to 60 wt. %, preferably in the rangeof 45 wt. % to 55 wt. %, preferably in the range of 48 wt. % to 52 wt. %of the total weight of the noble metal, metal oxide, and graphene oxide.In some other embodiments, the amount of graphene oxide in thesuspension is in the range of 0.1 wt. % to 10 wt. %, preferably in therange of 0.5 wt. % to 8 wt. %, preferably in the range of 1.0 wt. % to 6wt. %, preferably in the range of 1.8 wt. % to 4 wt. %, preferably inthe range of 2.0 wt. % to 3 wt. % of the total weight of the noblemetal, metal oxide, and graphene oxide.

The mixture is heated at a temperature in the range of 100-200° C.,preferably in the range of 110-180° C., preferably in the range of120-160° C., preferably at about 150° C. for a time in the range of 10to 20 hours for a time in the range of 10 to 20 hours for a time in therange of 5 to 30 hours, preferably in the range of 10 to 25 hours,preferably, about 15 hours. The noble metal nanoparticles ornanocomposite are filtered, washed, and dried at a temperature in therange of 50-120° C., preferably 60-100° C., and preferably 70-80° C.

Another aspect of the invention is directed to a method ofphotodecomposition of organic compounds comprising contacting thephotocatalyst of the invention with an aqueous solution of the organiccompounds and irradiating the solution with sunlight for a time requiredfor the decomposition of the organic material. The photochemicaldecomposition reaction may be monitored by well-known methods such asUV-Vis spectroscopy, HPLC, visual photo-bleaching of the aqueoussolution, and the like. In some embodiments of the method, the organiccompound is a dye such as but not limited to methyl orange, methyleneblue, indigo, tyrian purple, and the like that contaminate water. Thephotocatalyst of the invention is an effective catalyst for thedecomposition of such dyes by sunlight. The irradiation time requiredfor the decomposition of a dye in water varies with the chemicalstructure of the dye. In some embodiments, it is at least 0.5 h, 1.0, h,1.5 h, 2.0 h, 2.5 h, 3.0 h, 3.5 h, 4.0 h, 4.5 h, and 5.0.

A photodecomposition step can be incorporated as step in a process ofpartially or fully purifying contaminated water with water solubleorganic compounds such as dyes for human and animal consumption orreleasing the water into the environment. The contaminated water may beobtained from any industrial source such as, but not limited to chemicalplants, textile manufacturing plants, petrochemical plants, sewageplant, rain runoff, and the like. The process may include, but notlimited to filtration, a photodecomposition step, and desalinationsteps.

EXAMPLE 1

Preparation of Date Seed Extract:

The date seeds were collected from Al-Hasa city (Saudi Arabia). Washedand dried seeds were grinded using automatic grinder. Fine date seedpowder (30 g) was suspended in 100 ml distilled water and the resulteddate seed suspension was heated and stirred for 3 hrs. at 80° C. Theresulted date extract suspension was then filtered to remove the solidparticles and then stored and refrigerated.

The solid date seed extract was obtained by evaporating water from theprepared aqueous date seed extract using rotary evaporator at 20 mbar at25° C. Shiny red crystals were obtained after water evaporation. The XRDand FTIR of the as obtained date extract crystals was measured. FIG. 1Ashows the XRD pattern of the date extract crystals. Sharp band at 19.7°is observed which might be resulted from some bioorganic compoundspresent in the date seed extract. FIG. 1B shows FTIR spectrum of theseed extract. The peak at 3391.9 cm⁻¹ can be attributed to stretchingvibration of O—H/N—H groups. The peaks at ˜2925 cm⁻¹ are due to C—Hstretching vibration. The presence of peaks at 1612 and 1745 cm⁻¹ may beassigned to aromatic C═C stretching vibration. It is known that dateseed extract is rich in bioactive flavonoids and polyphenolic compoundssuch as, but not limited to hydroxycinnamic acids, flavonols, flavones,gallic acid, and flavan-3-ols (FIG. 2 ).

EXAMPLE 2

Synthesis of Au/ZnO/rGO:

Graphene oxide was prepared performing modified Hummers method [Hummerset al. “Preparation of Graphitic Oxide” J. Amer. Chem. Soc. 80 (1958)1339-1339, incorporated herein by reference in its entirety]. Au/ZnO/rGOwas synthesized by dissolving 5.0 g zinc acetate dihydrate (99 %purity)in 100 mL of an aqueous suspension of GO (2 mole % of the molar amountof zinc acetate). 5 mole % of chloroauric acid (HAuCl₄) of the molaramount zinc acetate was dissolved in the zinc acetate solution andfurther stirred for 30 min. Date seed extract (50 ml) was addeddrop-wise to the above solution with stirring and allowed to stir for 1h. The resulted suspension was transferred into an autoclave and heatedat 150° C. for 15 h. The water was evaporated using rotary evaporator at20 mbar at 25° C. and dried in oven at 70° C. for 3 h. The resultingresidue was washed and calcined at 400° C. for 30 min to ensure theremoval of any remaining organic materials. ZnO/rGO nanocomposite wasprepared by the same method without adding auric acid. Moreover, pureZnO nanoparticles were synthesized by a similar process but in absenceof GO and auric acid. In order to compare the effect of date seed (DS)extract, all nanomaterials were prepared in absence of DS extract usingNaOH. The samples have been denoted as Z, ZG, AZG, respectively, forZnO, ZnO/rGO, Au/ZnO/rGO, respectively, prepared in the absence of DSEand Z DS, ZG DS, AZG DS, respectively, for ZnO, ZnO/rGO, Au/ZnO/rGO,respectively, prepared in the presence of DSE. The prepared particleswere characterized by XRD, Raman, SEM and TEM and EDX.

EXAMPLE 3

Characterization of ZnO, ZnO/rGO and Au/ZnO/rGO Composites:

XRD measurements were performed using a Cu-K x-ray and 40 kV. FTIRmeasurements were recorded on IR AFFINITY-1 Shimadzu's. Scanningelectron microscopy (SEM) images were taken using Hitachi S-4700.Transmission electron microscopy (TEM) analysis was performed on a JeolJEM 2100 at 200 KV. Samples were mounted on 300 mesh Copper Grids coatedwith holey Carbon film (AGAR C062/C). A single drop was placed on thegrid and blotted from the underside to leave the particles on the grid.Samples were allowed to air dry for 24 hours prior to analysis. EDXanalysis was performed using an Oxford Instruments X-maxn 80 andanalyzed using Aztec software.

XRD patterns of the nanocomposites are shown in FIGS. 3A and B. The XRDpattern of GO shows 2θ band at 10.8°, which is assigned to GO. The XRDpattern of ZnO and ZnO/rGO nanocomposite prepared in the absence of DSE(Z and ZG) and in the presence of DSE (Z DS and ZG DS) show thecharacteristic diffraction peaks at 2θ=31.7, 34.29, 36.21, 47.48, 56.53,62.7, 66.30, 67.86, 69.00, 72.46, 76.86 are attributed to [100], [022],[102], [110], [103], [200], [112], [201], [004] and [202] planes,respectively, assigned to the crystal planes of wurtzite hexagonal phaseZnO (JPCDS 36-1451). The XRD pattern of Au/ZnO/rGO nanocomposites eitherin the absence or in the presence of DSE (i.e., AZG and AZG DS,respectively) exhibits four identical diffraction peaks at 2θ=38.2°,44.5°, 65.6° and 78.6°, respectively, which are corresponding to the[111], [200], [220] and [311] respectively, of metal gold, respectively,(ICDD No. 4-0783), in addition to the diffraction peaks for ZnO. Lowerintensities of the peaks are observed for the samples prepared in thepresence of DSE. The observed variation in the peak intensities of XRDpatterns may be due to difference in the particle size distribution indifferent samples. The two different set of diffractions observed due topresence of ZnO and Au in AZG and AZG DS samples which indicating theformation of composite system with successful loading of Aunanoparticles. Furthermore, the characteristic diffraction peaks at2θ=10.1° for GO in ZG, ZG DS, AZG and AZG DS were not observed,indicating the transformation of GO to rGO sheets after hydrothermalreduction in presence of NaOH or date seed extract (DSE), with randompackaging [Zhang et al. “Graphene transforms wide band gap ZnS to avisible light photocatalyst. The new role of graphene as amacromolecular photosensitizer” Acs Nano. 6 (2012) 9777-9789].

Raman spectra of ZnO/rGO (ZG) and Au/ZnO/rGO nanocomposites (AZG) areshown in FIG. 4 . Week bands at 363, 434 and 533 cm⁻¹ are observed whichare assigned to ZnO nanoparticles. The band at 363 cm⁻¹ is generatedfrom second-order Raman spectrum arising from zone boundary phonons ofhexagonal ZnO. The band at 434 cm⁻¹ is assigned to non-polar opticalphonon E₂ (HI) vibration mode of ZnO in wurtzite structure and at 584cm⁻¹ corresponds to the E₁ (LO) mode of hexagonal ZnO, which isassociated with the oxygen deficiencies [Li et al. “Physical andElectrical Performance of Vapor-Solid Grown ZnO Straight Nanowires”Nanoscale Res. Lett. 4 (2009) 165-168]. Both spectra show the D and Gbands of graphitic carbon at 1340 and 1610 cm⁻¹ respectively arises fromthe defects created by the attachment of oxygen containing functionalgroups on the graphene's basal plane and G band assigns to thefirst-order scattering of the E_(2g) mode [Alomair et al. “Greensynthesis of ZnO hollow microspheres and ZnO/rGO nanocomposite using redrice husk extract and their photocatalytic performance” Mater. Res.Express 5 (2018) 095012]. No bands assigned for Au nanoparticles havebeen observed under the applied experimental conditions.

The SEM images of ZnO/rGO and Au/ZnO/rGO show agglomeration of ZnOparticles on rGO support (FIGS. 5A and 5B). The nanocomposite materialsmorphology, composition and dimensions were determined from TEMmeasurements (FIGS. 6A-6J). TEM images of ZnO prepared in the presenceof DSE (Z DS) are shown in FIGS. 6A-6C and show particles of ZnO withsize in the ranging of 30 to 40 nm. TEM images of ZnO/rGO nanocomposite(ZG DS) of FIGS. 6D and 6E show ZnO particulates with size ranging from30 to 40 nm anchored on the surface of the reduced graphene oxide. Aunanoparticles with an average size 15-20 nm are homogeneously welldistributed on the surfaces of ZnO nanoparticles and rGO sheets, seeFIGS. 6F and 6G. In comparison, the nanomaterials prepared in theabsence of DSE showing the formation of larger Au nanoparticles with thesize in range 40-60 nm disorderly distributed on the ZnO. The resultsindicate the role of DSE as both stabilizing and reducing agent for thesynthesis of the nanoparticle as well as in controlling the shape andsize nanoparticles.

The energy-dispersive x-ray spectroscopy (EDX) analyses are shown inFIGS. 7A-7C. The EDX spectrum of ZnO shown in FIG. 7A confirms thepresence of Zn and oxygen, while the EDX of ZnO/rGO shown in FIG. 7Bconfirms the presence of oxygen and Zn in addition to carbon. The Aupeak has been observed along with the Zn, O and C peaks in theAu/ZnO/rGO nanocomposite in the EDX spectrum shown in FIG. 7C.

EXAMPLE 4

Photocatalytic Activity:

The photocatalytic performance of the green synthesized ZnO, ZnO/rGO andAu/ZnO/rGO nanocomposites have been evaluated for the degradation of theorganic dye Methyl Orange (MO) as a model of water pollutant. Thephotocatalytic experiments were carried out using sunlight simulatinglamp (PT2192, 125 W). Photocatalyst (1 g/L) was dispersed in 100 mlfollowed by dissolving 20 ppm of MO dye. The resulting suspension waskept in the dark for 30 min under stirring to achieve equilibrium. Thesuspension was irradiated and liquid samples were taken before andduring the irradiation and filtered to remove the solid catalyst. Thedegradation efficiency of the green synthesized nanomaterials wasdefined in terms of the C/C₀ ratio, where C₀ and C represent the initialconcentrations of the dye and the concentration of the dye at time t,respectively. Also, the photocatalytic activities of the nanomaterialsprepared in the absence of DSE have been investigated for comparison.

The nanocomposite of Au/ZnO/rGO (both AZG and AZG DS) exhibit superiorphotocatalytic activity compared to ZnO/rGO (either ZG or ZG DS). FIG.8A shows UV-vis absorption spectral change of MO dye in the presence ofAZG and FIG. 8B shows the time course of MO degradation catalyzed by thenanomaterials described herein. MO dye alone exhibits very slowdegradation by visible light, while faster degradation rates wereobserved when ZnO nanoparticles prepared in the presence and absence ofDSE as a catalyst are used. In the presence of ZnO/rGO (ZG DS), moreefficient degradations have been achieved, resulting in the degradationof about 54% of MO dye in only 65 min. In contrast, about 95% of MO dyeis degraded in the presence of Au/ZnO/rGO (AZG DS) nanoparticles. Theenhancement of the visible light photocatalytic activity due to theembedded Au nanoparticles in ZnO/rGO is readily attributed to thecombination of sensitization of ZnO nanoparticles induced by theembedded Au nanoparticles and the synergetic effect between ZnO/rGO andAu nanoparticles. Such a combination enhances the electron-hole pairseparation. The side-by-side comparison the photocatalytic activities ofnanoparticles prepared in the presence of DSE and sodium hydroxide inthe photodegradation of MO has unexpectedly indicated that nanoparticlesprepared in the presence of DSE are more efficient catalyst than thoseprepared in the presence of sodium hydroxide. Such an unexpectedobservation may be attributed to the larger and non-uniform distributionof the nanoparticles in the composite produced with sodium hydroxide.

A schematic representation of a single step green synthesis ofAu/ZnO/rGO is shown in FIG. 9 . ZnO/rGO and Au/ZnO/rGO nanocompositeshave been synthesized by the single step green method using Zinc acetateas Zn (II) ions source. Reduction of GO and formation of ZnOnanoparticles was achieved simultaneously. When Au (III) ions arepresent in the synthesis, they are also reduced to Au atoms anddeposited on the surface of ZnO and/or rGO forming Au/ZnO/rGO. Such amechanism is supported by the TEM measurements and observation.Furthermore, the proposed mechanism of the effective photocatalyticoxidation of methyl orange on Au/ZnO/rGO nanocomposite is illustrated inFIG. 10 . Au/ZnO/rGO nanocomposite show higher activity compared toZnO/rGO, which may be owing to the visible light sensitization of ZnOinduced by the surface plasmon absorption of the embedded Aunanoparticles. The coupling of Au nanoparticles and ZnO/rGO produces anenhancement of the charge carrier separation and thus, enhancement ofthe overall photocatalytic activity.

The invention claimed is:
 1. A method of making a photocatalystnanocomposite comprising: preparing a date seed extract by grinding adate seed and heating the ground date seed in water at a temperature inthe range of 60 to 100° C. for a time in the range of 1 to 5 hours andseparating solids to form the date seed extract in the form of a liquid,preparing an aqueous suspension of zinc acetate, chloroauric acid, andgraphene oxide, mixing the aqueous suspension with the date seed extractto form a mixture, and heating the mixture at temperature in the rangeof 100-200° C. for a time in the range of 10 to 20 hours to form thephotocatalyst nanocomposite; wherein the photocatalyst nanocompositecomprises Au nanoparticles, hexagonal wurtzite ZnO, and reduced grapheneoxide (rGO): wherein the Au nanoparticles are present in an amount of 2mol. % to 10 mol. % based on the molar amount of the ZnO nanoparticlesand the rGO is present in an amount of 40 wt. % to 60 wt. % of the totalweight of the photocatalyst nanocomposite, wherein the Au nanoparticleshave a diameter in the range of 15 to 20 nm and are homogeneouslydistributed on the ZnO nanoparticles and on the rGO; the ZnOnanoparticles have a diameter in the range of 30 to 40 nm and arepresent on the rGO.
 2. The method of claim 1, wherein the photocatalystcomposite comprises rGO in an amount in the range of 48 wt. % to 52 wt.% of the total weight of the nanocomposite.
 3. The method of claim 1,wherein the photocatalyst nanocomposite comprises 5 mol. % of Au basedon the molar amount of the ZnO.